Age and source of different forms of carbon released from boreal peatland streams during spring snowmelt in E. Finland

Isotopic data are increasingly being used to quantify and understand the processes that control the release of carbon (C) from northern peatlands. We used δ¹³C and ¹⁴C measurements to investigate the source and age of different forms of aquatic C (DOC, POC and evasion CO₂) released from 2 contrasting (undrained v drained) forested peatland catchments at the end of the winter snowmelt period in boreal E Finland. The δ¹³CVPDB values of DOC (range −28.3 to −28.8 ‰) were generally more ¹³C depleted than evasion CO₂ (range −22.7 to −31.5 ‰) and showed no clear differences between the pre-flood, flood and post-flood periods. Both forms of C had evidence of bomb-¹⁴C (i.e. >100%modern), indicating that they contained substantial quantities of C fixed since the mid AD 1950s. However, DOC was ¹⁴C enriched compared to evasion CO₂, with ¹⁴C concentrations suggesting that, on average, DOC-C was ~5–6 years younger than evasion CO₂–C, with the most recently fixed C being released when temperatures were highest. POC was significantly depleted in ¹⁴C with conventional (uncalibrated) radiocarbon ages of 805–1135 BP. In contrast to other studies, the isotopic compositions of DOC and evasion CO₂ were very similar, suggesting a predominantly single and consistent C source (decomposition of soil organic matter; SOM) during the snowmelt period. Whilst we found no evidence to suggest that old (pre-bomb) C was being released at the end of the winter period, the drained site was associated with more ¹⁴C depleted and ¹³C enriched evasion CO₂, suggesting a closer link to the atmospheric CO₂ pool. Our isotopic data suggest that the various forms of C released to the aquatic system from these forested Finnish peatlands are closely related, largely unaffected by drainage and (at least in the case of evasion CO₂ and DOC) indicate strong connectivity between C cycling in the soil–plant–water system.